extruder stepper malfunction

Extruder stepper motor jitters but fails to engage forward or backward (during filament load and unload) and fails to print. I have made several very good prints, but machine quit feeding filament mid-print after about 6-hours. I would love to finish that print. Tried adjusting trim pot without success and have checked plugs and continuity on all lines. Stepper motor works perfectly when plugged into x and y axis lines, so I suspect the mother board is malfunctioning or the software is corrupted.

Is it possible to repair the problem or to possibly fix it by changing the setup in Marlin on pins.h to print head B and uploading using Arduino? I have no experience in that area, but suppose the way to do that is to upload only the code reassignment lines, or do I need to upload the entire block of pins.h code with the changes included? Any help or other suggestions would be deeply appreciated. Thanks!